The natural history of swine influenza is being studied retrospectively and prospectively by standard serologic and virologic techniques in swine herds in nature between epidemics, what factors are critical for the occurrence of epidemics in swine, and to make observations on the interchange of influenza virus between swine and man. Antibody surveys indicate that swine influenza virus (Hsw1N1) is endemic in swine on Oahu, Hawaii. Some herds have been found with antibody to current human strains of influenza virus (H3N2), but transmission of these strains in swine appear to be limited. To date, the data obtained are compatible with the hypothesis that swine influenza virus is maintained in certain herds, in the absence of lungworms, through chronic or latent infections in individual animals. Conditions critical for the occurrence of influenza epidemics in swine in Hawaii are still unclear.